1. Field of the Invention
The present invention relates to an optical system for projecting and forming a reduced size of an image of a fine pattern on a wafer using UV radiation as a light source.
2. Description of Related Art
In an image projecting system which reduces and projects a fine pattern of an integrated circuit (IC) or a large-scale integrated circuit (LSI), etc., onto a semiconductor substrate, UV radiation (light in the UV region), such as that emitted by an excimer laser, has been widely used in response to a demand to shorten the wavelength of light to be used in order to increase the density of the integrated circuit.
Conventional optical glass can not be incorporated in a UV optical system since the transmittance of conventional optical glass for visible light is insufficient for UV radiation. To this end, a lens made of quartz (SiO2) or fluorite (CaF.sub.2) which has a high UV transmittance is usually used instead of a conventional glass lens. However, a quartz or fluorite lens must be produced with high precision. Another problem is that it is difficult to correct aberrations caused in the lens, and difficult to evaluate the optical performance of the lens in manufacture, assembly or use. Namely, in a UV projecting optical system, which is provided to form an image that is diffraction-limited, the amount of aberrations remaining in the optical system must be decreased in proportion to the wavelength. To this end, it is necessary to enhance the production precision of the lens, for example, reducing profile irregularities, and adjusting the lens to improve precision. These two points are far more important for a lens for UV radiation, than a glass lens for visible light. In particular, if a highly precise adjustment can be carried out, the level of production precision would be reduced to some extent, thus facilitating the production of UV lenses.